Adding Storage to EC2 instance that’s already created

this is more related with linux

you need to extend storage

https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/recognize-expanded-volume-linux.html

please read the link and follow these steps

1. Identify the device name of the new volume that you attached to your instance. You can do this by running the command lsblk in your server's terminal. The output will show you a list of all the attached devices along with their device names and sizes.
2. Create a file system on the unallocated space using the appropriate command for your file system type. For example, if you're using an ext4 file system, you can use the command sudo mkfs -t ext4 /dev/xvdf1 to create the file system on the device named /dev/xvdf1.
3. Create a mount point directory for the new file system. This can be any directory in your server's file system that you want to use as the access point for the new storage. For example, you can use the command sudo mkdir /mnt/new-storage to create a new directory named new-storage under the /mnt directory.
4. Mount the new file system to the mount point directory using the command sudo mount /dev/xvdf1 /mnt/new-storage. This will make the new storage available at the /mnt/new-storage directory in your server's file system.
5. Optionally, you can configure the file system to mount automatically at boot time by adding an entry to the /etc/fstab file. For example, you can add the following line to the file: /dev/xvdf1 /mnt/new-storage ext4 defaults 0 0.

As mentioned in the other replies, the OS has to extend the file system. You've presented the larger storage to the instance. The instance itself "sees" the additional storage, but it lacks a file system. So, this is similar to seeing an additional partition on the disk because the file system only covers what you had before. See the other replies from the 2 other authors, based upon your OS to extend the file system.